"The original engineering done by Knight Piésold Ltd. accommodated a significantly lower water volume than the tailings storage facility reportedly held at the time of the breach," it said.

The firm, which also posted the 2011 letter to Imperial Metals, states that it highlighted at the time how important it was for them to monitor and operate properly such tailings “to prevent problems in the future."

Gerald MacBurney, a former foreman who worked on the tailings pond, told CBC News TV last week he had warned of a disaster in the making for two years. According to MacBurney, management kept building the dam higher and disregarded his requirements to shore up the dam with enough rock to make it safe.